Cancers achieve invasive growth by delivering critical factors into the tumor microenvironment (1), but the molecular mechanisms for the secretion of these pro-invasive factors remain largely unknown. One likely process involves vesicle exocytosis, whose role in tumor progression was first reported by Palmer and co-workers (2). They showed that ectopic expression of BAIAP3, a Munc 13-like effector of regulated exocytosis, enhanced the malignancy of cancer cells.
Key players in exocytic and endocytic membrane trafficking include the Rab GTPases, which serve as molecular switches oscillating between their GTP-bound active and GDP-bound inactive conformations. Rabs recruit specific protein complexes to elicit their biological functions (3-6); they are post-translationally modified by geranylgeranylation, which binds them to lipophilic membranes (7).
The secretory pathway can be divided into constitutive and regulated portions (8). In the constitutive pathway, release of vesicle content occurs at a constant rate, and vesicles do not accumulate to an appreciable extent (9). In contrast, regulated secretion involves two distinct steps. Newly synthesized proteins are first stored within vesicular structures and are then released upon stimulation (10). Certain Rab GTPases, referred to as secretory Rabs, control this secretory process; they include Rab26, Rab37, Rab3A/B/C/D, and Rab27A/B (11). Rab26 and Rab37 are thought to modulate secretion in specialized cell types, whereas the Rab3 and Rab27 subfamilies function as more generic regulators of secretion (12-16). The Rab27 subfamily has the highest homology (41-44%) to members of the Rab3 subfamily; Rab27A and Rab27B exhibit 71% identity at the amino acid level (17).
Rab proteins of the endocytic (e.g., Rab25, Rab23 and Rab5) (18-21) and constitutive secretory pathways (e.g. Rab8) (22) play significant roles in malignancy and Rab GTPases active in exocytosis/secretion could also be critical for cancer progression.
WO 2006/091776 discloses a method for predicting prostate cancer progression via determining the expression level of a set of genes such as the gene encoding for Rab27. WO 03/004989 further discloses that Rab27B is over-expressed in breast cancer cells and that Rab27B can be used to screen for the presence of breast cancer. Hendrix et al. (40) further indicates that Rab27B is a potential biomarker in breast cancer progression. US 2007/0218512 indicates that human matrix metalloproteinase 26 (MMP 26) can be used as a biomarker, possibly in combination with an additional biomarker such as Rab27B, for evaluating the prognosis of cancers, among them ER-positive breast cancers. Recently, Wang and co-workers showed that up-regulation of Rab27A further enhances the already established invasive and metastatic phenotypes of the human breast cancer cell lines MDA-MB-231 and MDA-MB-435 (23, 36). In these models, Rab27A had a peri-nuclear and non-cytoskeleton associated localization pattern, suggesting a non-secretory function of Rab27A in MDA-MB cell lines. Human Rab27A and B are further structurally very similar and are functional homologues with respect to melanosome transport (35).
ER positive breast cancers, which comprise the majority of breast malignancies, carry a better prognosis for disease-free survival and overall survival than ER-negative breast cancers (37). Nevertheless, some ER-positive breast cancers are more invasive and tend to metastasize more frequently than other ER-positive tumors. A low degree of differentiation and the presence of metastasis in the axillary lymph nodes are typical characteristics. The underlying reasons for the more aggressive character are poorly understood. In this regard, Wright et al. (41) recently demonstrate in FIG. 3 of their publication that a lower level of Rab27B expression was found in ER-negative breast cancer tissue samples compared to the Rab27B expression in ER-positive samples which suggests that relatively increased Rab27B expression correlates with a positive outcome of disease.
However, it is currently still unknown which biomarker can be used to evaluate the prognosis of patients with estrogen receptor-positive breast cancer and especially the subset of patients with ER-positive breast cancers which are more invasive and tend to metastasize more frequently, or, can be used as target for drugs to treat the latter subset of patients.
Thus, needed in the art are reliable methods for stratifying, prognosing and treating the ER-positive breast cancers which are more invasive and tend to metastasize more frequently than other ER-positive tumors, as well as predicting treatment outcomes.